Regeneration of Southern Rata (Metrosideros Umbellata) and Kamahi (Weinmannia Racemosa) in Areas of Dieback 1

Pacific Science (1983), vol. 37, no. 4 © 1984 by the University of Hawaii Press. All rights reserved Regeneration of Southern Rata (Metrosideros umbellata) and Kamahi (Weinmannia racemosa) in Areas of Dieback 1 R. B. ALLEN2 and A. B. ROSE2 ABSTRACT: During the 1950s, dieback of southern rata (Metrosideros umbel/ata)umbellata) and kamahi (Weinmannia racemosa) was documented in the conifer/ hardwood forests of the Kokatahi and Fox catchments in Westland, New Zealand. Thirty years later, southern rata was usually absent as a live tree in dieback stands. Kamahi, although absent from the canopy in the Kokatahi, had partially recovered at Fox, where it was still a dominant canopy species. Regeneration studies on a range of sites indicate that in the short term, kamahi and Quintinia acutifolia will become the structurally dominant canopy species in many of the dieback stands, and southern rata will at best be a minor component. DIEBACK OF INDIGENOUS FORESTS is a wide gitude, 43°30' S latitude. Both areas are spread phenomenon in New Zealand; it oc within the West Coast "beech gap," which ex curs in many ofthe important genera, such as tends in latitude from approximately 42°50' S Eodocarpus, Nothofagus, Metrosideros, and to approximately 43°40' S.. Between these Weinmannia. Dieback of southern rata (M. latitudes, Nothofagus species are absent and umbel/ata)3umbellata)3 and kamahi (W. racemosa) is a the dominant canopy hardwood species are feature of the steepland conifer/hardwood southern rata, kamahi, and Quintinia acuti forests of Westland, where these species are folia. abundant, and in many other areas where Soils in this area are derived from fractured these species occur. It is generally believed schist along the alpine fault. They are that dieback of southern rata and kamahi is typically skeletal on steeper slopes, with principally due to browsing by the introduced increasing development on lesser slopes and brush-tailed possum (Trichosurus vulpecula podzol formation on gentle slopes. As phy Kerr.). The importance of other factors was siography is the result offluvial modification considered by C. G. R. Chavasse (1955, of formerly glaciated valleys, the forested unpublished) and more recently by Veblen slopes are steep, often greater than 30°, and and Stewart (1982) who support the hypo are subject to landslides. Such landslides are thesis that much of the apparently excessive usually in the form of debris slides or debris tree mortality is related to natural stand avalanches. Climate is characterized by high dynamics. annual rainfall and high humidity. Extensive The mortality and regeneration patterns in northerly airflows are usually accompanied dieback stands are described for two areas: by prolonged rainfall that may reach torren the Kokatahi catchment, at approximately tial intensities. Studies in adjacent areas 171 °10' E longitude, 42°55' S latitude, a tribu suggest rainfall reaches a maximum just east tary of the Hokitika River; and the Fox ofthe alpine fault, where it exceeds 1O,000mm catchment, at approximately 1700 00'E lon- per annum. Much of this area receives over 6000mm per annum (Griffiths and Mc I N.Z.F.S. No. I684/0DC 182.2. Manuscript accepted Saveney 1982). 5 October 1983. The introduction of browsing animals has 2 Protection Forestry Division, Forest Research Insti tute, P.O. Box 31-011, Christchurch, New Zealand. caused modification to these forests. Brush 3Nomenclature follows Allan (1961) unless stated tailed possums have been present in both the otherwise. Kokatahi and Fox catchments for over 40 yr. 433 434 PACIFIC SCIENCE, Volume 37, October 1983 Chamois (Rupicapra rupicapra L.) occur in kamahi and Q. acutifolia were abundant, but both areas. Red deer (Cervus elaphus L.) have occurred mainly on logs. Very few seedlings been present in the Kokatahi for over 40 yr, or saplings of southern rata were present. but are rare at Fox. Numbers have declined Intact stands of small-diameter trees oc markedly in the Kokatahi since the 1950s. curred on the steeper slopes forming the As a consequence of dieback, major escarpments of the Kokatahi River. These changes occur in the structure and composi contained southern rata, kamahi, and Q. tion of the affected stands. This paper ex acutifolia in the canopy. amines the changes that have resultedfrom dieback in stands dominated by southern rata Mid-Altitude Forests (450-650 m) and kamahi, and the resulting consequences Southern rata, kamahi, and Quintinia for maintenance ofthe structurally dominant acuiifolia were the dominant canopy species canopy species. The forests in each study area in stands before dieback occurred. Broadleaf are first considered in altitudinal classes based (Griselinia littoralis) became more abundant on the canopy species dominant before die with increasing altitude. back, followed by general comments on the It is in this zone that mortality has been area. These preliminary results are based on most extensive and complete. Figure 1· pre observations and some plot data. sents diameter size class distributions ofthe canopy species from a quadrat established in a representative stand in 1982. Within this KOKATAHICATCHMENT quadrat, all southern rata stems were dead. Dieback in the Kokatahi first became ap This included small single-stemmed trees, the patehtin the lat~ 19408. The-mbstex:tensive occasiortalffluchlatgeritH:lividulfl; -and some areas ofdieback in Westland were considered multistemmed trees. One etiolated seedling to be those occurring on the north bank ofthe was the sole representative of southern rata Kokatahi River (J. M. Hoy 1955, unpub on 180 m2 ofseedling plots measured on logs, lished). On some ridges between 300 and on tree fern trunks, and onthe ground within 700 m altitude, it was estimated that 75% of the above quadrat. Most of the remaining the original canopy, including southern rata dead stems were kamahi or Quintinia acuti and kamahi,was dead (L. W. Boot 1955, folia. There were no live stems ofkamahi, but unpublished). Areas ofup to tens of hectares Q. acutifolia was abundant in the smaller from the valley bottom to the upper al diameter size classes. Thedense shrub tier was titudinallimit of southern rata were affected. rich in species, of which Pseudowintera colorata and Carpodetus serratus were the Low-Altitude Forests (300-450m) most abundant subcanopy species, and Q. acutifolia was the dominant canopy species The hardwood canopy species dominant (Figure 2). Many tree ferns· were present, par before dieback occurred were southern rata, ticularly Alsophilasmithii (Hook. f.) Tryon. kamahi, and Quintinia acutifolia. The coni Seedlings. of kamahi and Q. acutifolia were fers rimu (Dacrydium cupressinum) and miro growing terrestrially as well as epiphytically (Podocarpus ferrugineus) were also present, on logs and tree fern trunks. rimu being limited to low-altitude stable sites. Dieback resulted in the loss of southern High-Altitude Forests (650-750 m) rata, kamahi, and much of the Quintinia acutifolia from the canopy. The podocarps Southern rata and broadleaf trees formed remained healthy. In stands affected by die an open-canopied forest, often on slopes of back in the canopy, the more abundant sub less than 10°. canopy species were Carpodetus serratus and After dieback, all the southern rata trees Pseudowinteracolorata. Saplings and seed and many of the broadleaf trees died (Figure lings of podocarps were present, particularly 3). The only live trees in the canopy were near parent trees. Seedlings and saplings of broadleaf, many of which had only partial I#'M Regeneration of Southern Rata and Kamahi-ALLEN AND ROSE 435 Dead Stems 100 Metrosideros Others 80 umbel/ata n-79 n -24 60 40 20 Ul E 0 G~I b/l~J ...QI 5- l C/) 14 134 304 -0 QI Live Stems ~ 100 Others ...c Quintinia Griselinia KOKATAHI 1 QI 80 acutifolia litt()ralis n-13 ~ n-625 n-46 Altitude: 500m ~ 60 Aspect: 2500 0 40 Slope: 25 Size: 54m x 36m 20 0 S S 5- 14 FIGURE 1. Percentage of live and relatively intact dead stems for canopy species in diameter size classes: n = number of stems> 104m tall; S = saplings, for stems < 5cm d.b.h.o.b. (diameter at breast height over bark) but > 104m tall; and then in lOcm d.b.h.o.b. size classes. crowns. The most abundant species forming Annual ring counts from disks and increment the shrub tier were Pseudowintera colorata, cores suggest that many of the subcanopy Coprosma ciliata, and Myrsine divaricata. trees and shrubs were present before dieback. Broadleaf and many subcanopy species were Most ofthe Q. acutifolia had been established regenerating on the bases of dead trees and after dieback, but individuals that predated fallen logs. However, seedlings of southern the dieback showed more rapid growth over rata were rare. the last 30 yr. Quintinia acutifolia was flower Thirty-six 400 m2 quadrats established in ing at ages of less than 20 yr, and because of 1979 over a range offorested sites throughout its abundance in the developing stands was in the Kokatahi catchment showed little ad a good position to occupy vacant sites. vance growth of southern rata in open- or The dieback stands described in this study closed-canopied stands, although seedlings were of the earliest described mortality. Die and saplings of kamahi and Quintinia acuti back in southern rata and kamahi has oc folia were relatively abundant. Species highly curred at other times, and extensive areas of palatable to red deer were present within the recent dieback were present on the south side browse tier, indicating that red deer were of the Kokatahi catchment. Some of these unlikely to be limiting the regeneration of trees were not yet dead, although most ofthe southern rata, although this may not always foliage had been lost. have been so. An earlier report considered On recent landslides within areas of die that red deer checked the establishment of back, regeneration of kamahi and Quintinia southern rata and kamahi in dieback stands acutifolia was usually dense.

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